1. Field of the Invention
The present invention relates to a seismic prospecting method and apparatus which use an improved vibratory signal sweep, and more particularly to a seismic prospecting method and apparatus which use a vibratory signal sweep which is formulated in consideration of attenuation and signal scattering characteristics of the particular earth formation target with which the vibratory signal sweep is used.
2. Description of the Prior Art
The use of vibratory apparatus in seismic prospecting is well known in the art. Commonly, a vibratory sweep is used in such apparatus to vibrate the earth. The sweep typically lasts for 20 to 30 seconds, during which time the instantaneous frequency of the vibratory oscillating signal varies linearly and monotonically with time, usually from a first lower frequency f.sub.1 to a second higher frequency f.sub.2. The amplitude of the oscillation signal remains substantially constant over the duration of the sweep, but it is preferably linearly tapered near the beginning and ending of the sweep to avoid signal overshoots and to facilitate signal processing of the sweep wave reflected from subsurface formations.
While this type of vibratory sweep is considered good for general purpose seismic prospecting, it has limitations since the various frequency components of the sweep are affected differently by the signal scattering and absorption effects of the earth formation. Commonly, when the vibratory sweep signal is received by a receiver after being reflected by subsurface formation conditions, the higher frequency components thereof are scattered and attenuated to a greater degree than the low frequency components. For data processing purposes, the received reflected signal waveform should have as flat an amplitude characteristic as possible. However, processing the received signal to yield a flat amplitude spectrum by amplifying (equalizing) the higher frequency components of the reflected wave also undesirably increases a background noise component at the higher frequencies, so that the noise component increases in amplitude, with increasing frequency of the vibratory sweep.
Many variations to the linear vibratory sweep discussed above have been proposed to control the amplitude of the vibratory sweep signal throughout the frequency spectrum of interest. See, for example, the paper "Signal Design In The `Vibroseis`.RTM. Technique" by Goupillaud, published in Geophysics, Vol. 41, No. 6, December 1976, pages 1291-1304. However, these variations are based on changing the vibratory signal sweep in accordance with a predetermined mathematical function without regard to the specific characteristics of the subsurface formation with which the vibratory apparatus is used. Accordingly, although one or more of the known vibratory sweep signal patterns may work well with one type of subsurface formation, they will not necessarily work well with other formations having different signal scattering and attenuation characteristics.